Ok, so everything has been doing ok for a while, but today we did a 50-60% water change, and it killed our two firefish. Everything else seems to be doing ok, the shrimp are doing better, but for some reason the firefish both keeled over. I got a new test kit for my levels, and they are all ok, pH, nitrate/ite, ammonia, all just fine. Salinity and temp are ok too. What is going on???

Is there anything I could be checking in my tap water that might be causing this??